The present invention relates to a method and an apparatus for switching a field-effect transistor from an xe2x80x9coffxe2x80x9d to an xe2x80x9conxe2x80x9d state, or vice versa, the switching being effected by the application of a control voltage suitable for the switching to an RC element connected upstream of the gate terminal of the field-effect transistor.
Methods and devices of this type are used, in particular, in large driver stages (for example in output drivers) in integrated CMOS circuits, in order to prevent an abrupt changeover or switching of field-effect transistors that switch large amounts of power and to prevent the associated disadvantages such as, the occurrence of supply voltage fluctuations and electromagnetic interference, in particular.
A field-effect transistor with an RC element connected upstream of the gate terminal is illustrated in FIG. 3. In this case, the field-effect transistor is designated by the reference symbol T, the gate terminal of the field-effect transistor T is designated by the reference symbol G, the resistor of the RC element is designated by the reference symbol R, the capacitor of the RC element is designated by the reference symbol C, and the terminal via which the control voltage which controls the field-effect transistor T is applied to the gate terminal G thereof is designated by the reference symbol CTL.
The effect of the field-effect transistor being driven by an RC element connected upstream of its gate terminal is that the voltage at the gate terminal of the field-effect transistor changes relatively slow and does not change abruptly, which means that the field-effect transistor also changes states relatively slowly.
However, providing RC elements is relatively complicated, since RC elements having large time constants, in particular, occupy a relatively large area on the integrated circuit, which understandably constitutes a considerable disadvantage.
The present invention is based on an object, therefore, of providing a method and apparatus such that an abrupt changeover of the field-effect transistor can be reliably prevented with minimal technical outlay.
This and other objects are achieved according to the present invention by a method for switching a field-effect transistor from one state to another state including inputting a control signal having a first voltage level from a control input to a gate terminal of the field-effect transistor via an attenuating circuit and an RC element. The attenuating circuit and RC element are connected between the gate terminal of the field-effect transistor and the control input and are configured to apply control voltage for switching the field-effect transistor. Next, the field-effect transistor switch by at least temporarily attenuating the control voltage via the attenuating circuit to a second voltage level that is applied to the gate terminal via the RC element. The second voltage level is set to slightly exceed a threshold switching voltage of the field-effect transistor by a small amount and also being significantly less than the first voltage level.
According to another aspect of the present invention, an apparatus for switching the field-effect transistor is provided to switch the transistor from one state to another. The apparatus includes a control input to receive a control signal having a predetermined voltage level. An attenuation circuit is connected to a control input for receiving and attenuating the predetermined voltage level and, in turn, outputting a control voltage. Connected between the output of the attenuation circuit and gate terminal the field-effect transistor is an RC element for delivering the control voltage to the gate terminal with a predetermined time delay to switch the field-effect transistor. The control voltage is at least temporarily set by the attenuation circuit to an attenuated voltage level that slightly exceeds the threshold switching voltage of the field-effect transistor by a small amount and is significantly less than the predetermined voltage level.
Accordingly, the changeover of the field-effect transistor is carried out using a control voltage which at least temporarily only slightly exceeds the threshold voltage and has to be applied to the gate terminal of the field-effect transistor in order to be able to effect changeover thereof.
The effect that can be achieved by the present invention is that the voltage established at the gate terminal of the field-effect transistor changes particularly slowly in the region of the threshold voltage, even when the RC element used has only a relatively small time constant. This is because the voltage established at the gate terminal of the field-effect transistor is also the voltage established across the capacitor of the RC element connected upstream, and this voltage follows changes in the control voltage only gradually and becomes approximately the same voltage particularly slowly in the final stage.
As a result, an abrupt changeover of the field-effect transistor can be reliably prevented in a surprisingly simple manner and with minimal technical outlay.
Additional advantages and novel features of the invention will be set forth, in part, in the description that follows and, in part, will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The advantages of the invention may be realized and attained by means of the instrumentalities and combination particularly pointed out in the appended claims.